This invention relates to a method of and an apparatus for recovering energy in the manufacture of polymers, in particular high-pressure polyethylene, from monomeric gas, in which the monomeric gas is compressed and partially converted into a polymer in a reactor by polymerization.
The invention relates in particular to the use of this energy recovery method in the manufacture of polyethylene from ethylene monomeric gas by means of the high pressure method.
In this known high-pressure method the ethylene gas is compressed to high pressures (1000-3000 bar and more), in which only a moderate proportion of this monomeric gas is converted into polyethylene by polymerization in a reactor. The much larger proportion of non-reacted gas is re-circulated uneconomically and with a high energy consumption into a compression and decompression circuit; thus there is considerable energy consumption taking into account the moderate amounts of polyethylene obtained.
The known high-pressure method for the manufacture of polyethylene from monomeric gas takes place basically as illustrated in FIG. 1 of the accompanying drawings.
In the manufacture of polyethylene using the high pressure method, ethylene gas, C.sub.2 H.sub.4, supplied via a pipe 24 is compressed in stages to high pressures (1000-3000 bar and more according to the properties of the product required). The compression is carried out in a piston compressor S.
The high-pressure gas (stream m) subsequently flows into a reactor R, in which polymerization takes place by the injection of a catalyst. Only quite a small proportion (approximately 18-25%) of the total gas volume m in the reactor is converted into polymer and polyethylene. The remaining larger proportion which is unreacted remains as ethylene monomeric gas, which is linked to the polymer in a type of two-phase solution.
Under these high pressure and temperature conditions, at which the product is discharged from the reactor R into a pipe 1, it is not possible to separate the polymer and the ethylene gas.
To separate the polymer from the monomeric gas, the material stream m (polymer+monomeric gas) is decompressed by a choke valve 2 to a much lower separation pressure (approximately 250-300 bar). In a medium-pressure separator A. A considerable proportion of the amount of bonded ethylene gas is released (approximately 70% of m).
The separated, almost polymer-free monomeric gas (amount m-m.sub.o) is added to the ethylene gas (amount m.sub.o) to form the total stream m. The ethylene gas (amount m.sub.o) has been compressed by a primary compressor P to the separation pressure in the separator A. This gas volume m is compressed by the secondary compressor S to the reactor pressure and the process is re-started (high-pressure circuit R-1-2-A-24-S-R).
Therefore a large proportion of non-polymerized ethylene gas, in effect 3 to 4 times the amount of polymer obtained, is re-circulated in the high-pressure circuit.
The corresponding energy consumption for the compression, cooling and other processes is enormous in comparison with the moderate yields of polyethylene. In effect, the polyethylene obtained only corresponds to 18-25% of the total energy consumption. Most of the energy is used to re-compress the monomeric gas which has not been converted into polymer to the reactor pressure at the compressor S. Therefore the energy content of the material stream m is again lost without being used during the choking at the valve 2.
In this respect, the choking at valve 2 again effects an increase in the temperature of the material stream m causing a detrimental effect on the separability of the polymer and monomer. This requires cooling and corresponding expenditure of more energy.
The polymer in the medium-pressure separator A remains bonded to approximately 10% of the monomer gas. The amount m.sub.o (polymer+monomeric residual gas) of the stream of material discharged into a pipe 11 is further decompressed to a low separation temperature in a low-pressure separator A' via a choke valve 12. The degasified polyethylene (amount m.sub.p), discharged through a pipe 21, is suitable for further processing.
The separated residual ethylene gas in an amount m.sub.o -m.sub.p is mixed with freshly supplied ethylene gas (amount m.sub.l =m.sub.p) (Process A'-/22-23) and compressed by the primary compressor P to the pressure in the pipe 24. This gas volume m.sub.o (approximately 30% of the amount m) is mixed at 24 with the monomeric gas from the separator A to obtain the total amount m.
The low-pressure process (11-12-A'-23-P-24) includes a further expenditure of energy caused by the energy loss resulting from the choking at valves 2 and 12, because the proportion of residual gas must also be re-compressed from the low pressure at the separator A' to the reactor pressure by compressors P and S. The material stream is heated by the choking at valve 12, having the effects discussed above.
The manufacture of polyethylene by the high-pressure method is therefore carried out with an unsuitable degree of energy consumption, the particular causes of which are as follows:
the polymerization process takes place at high pressures with considerable energy expenditure;
the amount of polyethylene which is obtained from the entire gas volume m used in the process is quite small;
the polymer and monomeric gas are not separable at high pressures and temperatures and it is exactly at this point that the complete energy content (in the line 1) is present;
the necessity of decompressing the material stream m to low pressures to separate the polymer and the monomeric gas;
this decompression is carried out with choke valves (2,12) and the stored energy is unused and therefore lost;
the material stream is heated by the choking at the valves 2 and 12, which makes separability at higher pressures more difficult and necessitates additional cooling energy.